Note: This prevents the installation of the package systemd-shim as well. However this is a emulation of systemd functions, so you can install software which depends on systemd. So if you get "Package systemd-shim is not available, but is referred to by another package." you might temporary disable the three lines in the preferences.d/systemd file.

In the case your system uses multiarch (mixed 32 and 64bit packages), you need to pin the 64bit version of systemd too. Otherwise apt-get could still install it as dependency later.